In the September issue of Apollo, there's a new theory about one of the National GalIery's most famous paintings, Jan van Eyck's... ha! I must be careful here. Most people call it the Arnolfini Wedding, and that is largely because of a celebrated, but evidently wildly unsound, article by Erwin Panofsky in 1934, which proposed that the picture records a clandestine wedding to which the artist himself was witness. His signature on the wall above the convex mirror, which reads in Latin "Jan van Eyck was here", was ingeniously supposed by Panofsky to be in an elaborate legal script.

Panofsky's theory was not the first attempt to answer the questions raised by the painting, and the "simple solution" proposed by Margaret L Koster in Apollo will undoubtedly not be the last. Many people for instance have thought that the woman, who holds up the train of her dress with her left hand while placing her right hand in her husband's proffered left, is pregnant.

In previous centuries, when the panel was in a frame bearing a quotation from Ovid to do with promises made in love, the work was taken to depict a man making a vow (which he did not intend to keep) to a woman he had got in the family way. However, a few years ago students frorn Wimbledon College made a serious attempt to reconstruct the dress that the woman is wearing. From this you could see that, if you try to stand as the woman is standing, and you lift the train of the dress in the same manner, you too will look pregnant.

Nevertheless, childbearing may be a part of the subject. In the background, partly visible, is a high-backed carved chair, with a figure clearly intended as St Margaret, the patron saint of the child-bearing woman, and on the floor by the bed there is a carpet. This may be taken as an indicator of social status, but it has been noted that there was a convention in the Burgundian court that women of a certain rank had a carpet placed in front of the bed at the time of lying in - that is, giving birth.

The question is whether such details as this are to be taken as part of the symbolism of the picture, and whether there is a meaning to the whole ensemble, clear once to the painter's contemporaries, but obscure to us. In order to begin to make our minds up about such questions, we have, as it were, to make an inventory of what is in the room, and that in itself turns out to be a major task.

Lorne Campbell, who wrote the catalogue of Fifteenth Century Netherlandish Paintings for the National Gallery, devotes 35 pages and more than 300 footnotes to this painting alone, and a good part of that is devoted simply to explaining what it is we see. What we take to be a bedroom, but should more properly be called a reception room, turns out to be a somewhat puzzling construction, about whose contents the artist changed his mind, or developed his ideas, as the painting progressed. The mirror, for instance, changes size and shape. The chandelier may not have been part of the original idea. And so on.

There were several Arnolfini in the Netherlands at around the date of the painting, 1434. If the man is Giovanni di Nicolao Arnolfini, as Lorne Campbell thinks, then his wife was dead by the time Van Eyck signed the double portrait. Campbell suggests that the woman is a putative second wife.

Margaret Koster's theory is that one need not posit a second wife. The woman in the portrait may be already dead. "In the ritual practices of the period," she tells us, "when visitors were received into the home, they were brought into the principle chamber of the house where the new mother, the dying, or the recently deceased were to be found lying in state on a hung bed". Supposing Arnolfini"s wife had died in childbirth (she died at the age of 20), the carpet in front of the bed would allude to this fate. The praying figure of Saint Margaret could be taken as interceding for the soul of the departed. Immediately above the woman's hand there is a carved wooden monster, perhaps presaging her death.

The convex mirror is framed with 10 roundels depicting the passion and death of Christ. The scenes of the living Christ are on the man's side; the scenes of his death and resurrection are on the woman's. And in the brass chandelier there is a lighted candle on the man's side, and on the woman's - at least this is certainly what I see - there are the remains of a candle that has guttered out.

The astonishingly convincing dog at the couple's feet doesn't look like the dogs one finds on carved tombs, where it symbolises faithfulness. But that is a function of the difference between painting and sculpture. The symbolism would be the same. The green dress of the woman (neither costume is such as one would wear about the house) symbolises love. The man's garb would betoken grief.